Logarithmic photometer



M. H. SWEET LOGARITHMIC PHOTOMETER- Aug. z, 1949.,

Filed Dec. 30, 1944 0 wwwwwwmwuuma y may Patented Aug. 2, 1949 `signor to General Aniline Qorporatio,

New York, N. Y., a corporation of Delaware) Application December so, 1944, serial No. 570,627

s clams. (ci. 2501er) This inventionv relates tovelectron multiplier tube measuring circuits, and more particularlyto novel circuits for prOviding a, direct indication upon a uniformly graduated meter scale vork electrical magnitudes Which vary exponentially.

In numerous mensuration Operations, a much desired object is to obtain a direct indication of electrical values which, are inherently a logarithmic function of certain yphysical or electrical phenomena. Atypical example is the measurement of the density of a photographic lm. In the usual measuring .instrument for determining the value of the lm density, thelight luxtrans.- mitted through the nlm is directed upon a suitable photo sensitive device, `such as a photo-emissive vacuum tube. The output of the phototubeis then amplified, and the relative current or voltage values thereof are indicated upon a suitable current or voltage indicating meter. The radiant :dus incident upon the phototube is aninverse logarithmic function of the density of the lm. Since the output of a. phototube is generally linearly related to input excitationthe indicating meter must be provideydlwith a scale graduated logarithmically, if it is desired to read density directly. As is known, a logarithmic scale is nonuniformly graduated, the indicia thereof being crowded near one end of the scale and being widely spaced nearthehopposite end thereof. The non-uniformity of the scale graduations adversely affects the accuracy and facility of the meter readings.

Various expedients have been proposed for btaining direct indications of density on a meter having a substantially uniformly Vgraduated density scale. Among such expedients, cut pole pieces for the magnetin the. eld ofY which the coil of the meter rotates have been used to vary the sensitivity of responsethereof over different portions of the scale. l Theresults obtainable by such expedients havev been generally unsatisfactory. In my United ,StatesPatent 2,405,716', issued August 27, 1946, for .Direct reading densitometer, there is described and claimed a suitable electronic measuring circuit for obtaining direct readings upon a uniformly graduated meter scale of the density of aphotographic nlm. In such circuit, a logarithmic amplifier is provided between the output of a phottube and the indicating meter. The parameters of the amplifier are so selected that the meter indicates directly the density of the sample upon a uniformly graduated scale. The light ux incident upon the phototube is an inverse logarithmic function of the density of the sample, and the output current of the phototube is a direct function of the light incident thereupon. .By .interposing a logarithmically responsive amplifierA between the phtotube and the meter, therelative currentovving through the meter is inade'a direct function of the density ofthe, sample.. The described circuit has been satisfactory in, practican. c l

However, there haverecently been made available electron multiplier tubeswhich have a sensitivity` several,.thousandfolclv that of simple vacuum tubes used ,fon amplification in Iconventionalc'ircuits. Typical ofthe electron multiplier tubes, arethe electrostatically focusedphotomultiplier types, in which theoutput per unit of incident radiant. .energy, is ,several `thousandfold greater than thatvof 4simple photoemissive type vacuumV tubes. .i These.. multiplier tubes have opened a wide eld fOr-obtaining increased sensitivity` ofresponse.inmensuraton circuits.

The incorporationof such;multiplier tubes in circuits, having ,linear 4amplicationY presents no diculty. However, When it isattempted to incorporate,..such tube Aintoa logarithmically responsive circuit, considerable difculties are presented. In,my United. StatesPatent 2,417,623, issued MarchA, 1947, forjPhoto-electric electron multiplientube photometer circuits, are described circuits for. successfully utilizing a photomultiplier tube in a circuit fortdirectlywindicating density values u'pon .auniforml'y graduatedimeter scale usi-nga logarithmic amplifier betvveenthe multiplierV tube and .thelmeten V4The present invention. relates ..to..ano.tl1erL .type` of Acircuit, in which the .photomultiplier vtube,.is .made to function as a nonlinear..e.jg. logarithmicamplilier and may be followed bya lineanamplier stage which controls .themeter movement.

It is.. among :the objectszof this. invention to provide an electronic .measuring circuit for indicating, upon .a ullifornilygraduated meter scale, electricalquantities which vary in an exponential ratiosuch asa logarithmic ratio; to `provide a measuring `circuitiemploying an electronic multiplierltubeand .including means for varying the output ofthetube inlamanner. to obtain direct indicationsupon a uniformly vgraduated meter scale of electrical quantities Whichvary logarithmicallythe rate of excitation of the multiplier tube;` to provide av direct reading densitometer including japhotomultiplierv'tube and a meter havingau'niforml graduatedlinear scale for directly indicatingdenslity; "andmto provide novel circuit arrangements vfor robi:afrjlfiflfl. direct indications upon a uniformly graduated meterscale of functions .logaritlin1cally-related to the excitation of the meas'iiriiigcircuit, n

These and-,otherjob .ctsadvantagesvand novel features Qfthsinventlon v1.11 ...be apparent. from the.,folloWing V,description,and the accompanying drawing. lIn .the drawing-r. Y Y

Eig.- Lis ,atgraph-illustrating.the relation betweandensty .0i aisampleand output current in a CQIiYntQnalidenstylmeasurng electronic circuit having linear amplification.

Fig. 2 is a schematic circuit diagram illustrating one embodiment of my invention.

Fig. 3 is a schematic circuit diagram illustrating another embodiment of the invention.

Fig. 4 is a graph illustrating the relation between sample density and output current with the circuits shown in Figs. 2 and 3.

In accordance with the principles of the present invention, an electrostatically focused electron multiplier tube is arranged in responsive relation to a source of radiant energy. A sample, the density of which is to be indicated on a suitable meter having a uniformly graduated scale, is interposed between the tube and a light source. Means are provided for varying the potentials applied to the elements of the multiplier tube in such a manner that the meter readings are logarithmically related to the radiant energy incident upon the multiplier tube.

The means provided for controlling the amplication of the photomultiplier tube in such manner as to change it from a normally linear device to one which will have a non-linear response comprises an inverse feed-back circuit. This circuit is so arranged that in the feed-back path the output energy from the photomultiplier tube is applied to a control element which in turn modifies the effective operating potentials applied to dynode elements and thereby varies the effective amplification of the multiplier tube in an inverse ratio with the output thereof. The indicating meter may be connected either in the output circuit of a 'linear amplifier associated with the output circuit of the electron multiplier tube, or, alternatively, the meter may be connected directly in the anode circuit of the photomultiplier tube itself. Means are provided for introducing a bucking current into the meter circuit to establish a zero reference point for a current value denoting zero density.

While the invention will be particularly described as applied to photometer or densitometer circuits, it will be apparent to those skilled in the art that the invention is not limited thereto, and such description is exemplary only. The principles of the invention are applicable equally to electron multiplier devices which are either current or voltage excited to obtain direct readings of logarithmic functions. and to types of photometers other than densitometers.

Referring to the drawing, Fig. 1 is a graph illustrating the relation between the density of a sample, such as a photographicilm, and the relative values of the current through the indicating meter in a circuit employing a photo responsive device and a linear amplier. The density is an inverse logarithmic function ,of the amount of light passing through the sample and incident upon the photo responsive device, and the output current of the linear amplifier is a direct function of such incident light. The meter indicates the relative values of the output current of the amplifier. If the meter is to indicate density directly, its scale must therefore be logarithmic, and thus non-uniformly graduated. For instance, referring to Fig. 1, at a density value of two, the relative current through the meter is represented by the factor one; at a density value of 1.0, the relative meter current is represented by the factor ten; and at a density value of Zero, the relative current through the meter is one hundred times the current therethrough at a density value .of two. In other words, the relative currents through the meter for any given density are logarithmically related to each other. This '4 results in the solid line curve I0 shown in Fig. 1.

For purposes of increasing the accuracy and facility of reading the meter, it is highly desirable that the meter have a uniformly graduated scale. However, it is not possible to use such a uniformly graduated scale where the meter indicates density directly and a linear amplifier is used between the photo responsive device and the meter. The indications of a meter having a uniformly graduated scale indicating density directly are represented by the broken line ll of Fig. 1. In other words, to obtain direct readings of the density on a meter having a uniformly graduated scale, the output current through the meter must be increased at corresponding points by the difference between the solid curved line l0 and the broken line ll, or at any other vertical line intersecting the curve in the graph.

The present invention provides an arrangement for automatically varying the sensitivity of a photomultiplier tube in such a manner that the desired increase in the output current through the indicating meter will be substantially obtained at any given reading. In other words, with the circuits shown in Figs. 2 and 3, the output current through the meter at the density values given in Fig. 1 is increased from the values A, B, C, D, E and F tc substantially the values A', B', C', D', E' and F'. In the present invention, this is accomplished by controlling the potentials applied to the multiplier electrodes or dynodes of the electron multiplier tube in accordance with the output current of such tube.

An understanding of how this is accomplished may be had by reference to the following Table I illustrating the relation between sample density, flux incident upon the cathode of the multiplier tube, the desired meter reading, and the amplification of the tube.

From published data on one type of photomultiplier tube relating the output sensitivity `of the tube to the potential applied between the multiy plier electrode or dynodes, a set of dynode potentials may be computed which will provide the proper amplification factors as listed in Table II. These values are as follows:

Table II Density of Amplification Dynode Sample Factor Voltage Volts per stage 3. 0 1. 00 57. 5 2. 5 47 52 2. 0 2D 46 1. 5 078 40 1. 0 030 35 0. 5 011 30 0. 0 O04 25 It will be noted from Table II that the relation between the sample density and the dynode potentials is linear within close limits. Accordingly,

if the dynode voltages are linearly controlled as a function of thesainple density, Hthe amplication factor for providing'the "desired uniform meter reading may besimpl'y obtained. Figs. 2 and 3 illustrate circuits which operate in the manner to obtain the above results.

Referring to Fig. 2, Valight source such as an incandescent lamp l5 is arrangedin operative relation to a photornultiplier tubezgll, which may, for example, be ofthe RCA-E231 type. Cathode i6 of tube 20 is arranged to receive light from lamp i5 directed thereon'threugh aV sample l'I, the density of which is'to be measured and indicated by a meter 25 operatively. associated With the output circuit of tube Light from lamp l5 may, if desired, be directed through a lens I8, a filter 2i and Va trarispar'entl support 22 onto cathode it of tube 20. i

The operating potentials applied to cathode I6, multiplier elements or dynodesfZ'S and anode 24 of tube t@ are obtained from'a suitable power supply having terminalsy 21Tn andv v3| "with the polarity indicated.`

The voltage from the power supply is applied to the elementsl of the phdtmultipliertube 20 in such manner that Vtherei a'control tube in the supply current path as' well as a current indicating instrument; For this reason the positive terminal Vof the supply connectstofthe anode @d of the controlv tube 35 andthe cathod'ef36 thereof connects to a voltage divider consisting of a plurality of resistors 30 in series, the last one of which terminates at the vnflegative terminal of the supply. The junction pmi-itsv of resistors 30 connect to respectivedynode elements; 23 and the anode 2t of phototube 20conne'cts through a load resistance lll to the'junctionlpoint indicated with reference character 28, of the voltage divider and the cathode 30 of tube.3,5.

From this it is seen that the current in the e vonage divider elements 30,15 controuedby the control tube inasmuch as the dividerjis in effect the cathode resistor of this tube. In the circuit shown the control tube 3'5 is illustrated` as a tetrode for the sake of simplicity. A pentode may also be used or a triode providedit'hasthe required transfer characteristics. As Will be seen the control tube may havea dualfunction in that aside from controlling the effective dynode voltage it may also amplify currenty variations of the amplifier output circuit and. apply. them to the indicating instrument.

The control grid 31 of tube.35 is. connected to anode 2t of tube Z0 andthe screen grid38 connects to the positive terminal of a power supply shown here by a battery 40.' The negative ter.. minal of the battery returns to cathode 36. The resistor d2 and suitable source'vof potential such as a battery t3 in series therewithare connected in parallel with meter 25'. The purposeof this arrangement will be described hereinafter in considering the operation ofY the circuit'. In a practical example, resistor. 4'I mayhave a value of 50,000 ohms. The ten seriesv connected resistors 30 forming the voltage divider may each have a value of 13,000 ohms,` Where .the potential of the supply is 1000 volts.

The circuit shown in Fig. 2 operates. in the following manner. Control tube .35. controls thejcurrent flowing through.resistorsxand thus the effective operating` potentialb applied. between cathode i6, dynodes 23and. anode@ of tube-20, The potential of controljgridS-'lof -tubeiii Vwith respect to cathode 36 is governed by the voltage drop acrossthe anode, load.k resistorJ 4l of the phototube 20. When the output current in the anode load resistor lll of phototube 20 is high, corresponding to low densities of sample I 1, the voltage drop across resistor 4I will also be high. This creates a relatively large negative bias on control grid 3l of tube 35. The relatively large negative bias decreases the amplification of tube 35 and thereby reduces the current flow through Voltage divider 30. The decreased current flow through voltage divider 30 results in a corresponding reduction in the potentials applied ben tween the elements of photomultiplier tube 20.

It Will be noted that the effective operating potentials applied to the elements of phototube .20 are reduced as a linear function of the output current of the device. Thus, the linear relations between the density of sample l1 and the potential applied to the elements of tube 20, indicated as desirable in Table II supra, are obtained.

The output current from plate il@ of tube 35 is approximately a linear function of the potential variations of control grid 31 of the tube over a considerable portion of the grid potential-anode current characteristic curve. The anode circuit of tube 35 includes the meter 25. Accordingly, the latter indicates the output current of tube 35. The output current of tube 35 varies linearly with variations in the output current of device 20. At controlled dynocle voltages the output current of device 20 is substantially a logarithmic function of the light ux incident upon its cathode I6 through sample l1. The incident light flux is an inverse logarithmic function of the density of the sample. Consequently, the indications of meter 25 are a direct function of the density of sample l? and a uniformly graduated scale in ay be used for the meter.

On any uniform scale densitometer, innite density Would theoretically yield an infinite positive or negative response. For obvious reasons, only a nite range of densities can be measured iby the meter. Accordingly, a bucking current is introduced into the meter circuit so that the meter will read the maximum measurable density at one extreme of its scale. This bucking current is introduced by the battery 43, through the series resistor t2 and opposes the anode current of the tube 35 at the particular density value chosen to be represented by zero meter indication.

Since the tube 35 is not only a control tube but functions also as substantially a linear amplifier of the output voltage of phototube 20, it will be evident that the indicating meter may likewise be placed in the output circuit of phototube 20 instead of being placed in the output circuit of tube 35 if the additional gain is not required. Such an arrangement is illustrated in Fig. 3 wherein elements corresponding to those in Fig. 2 have been given the same reference characters primed. In the circuit of Fig. 3, one meter terminal is connected to anode 24 of device 20', and the other terminal is connected to the anode load resistor 4l'. Anode d4 of tube 35 is connected to terminal 3l of the current source. Terminal 27' of the current source is connected to terminal 2B' of voltage divider 3d'. Terminal 2a of voltage divider 30 is connected to cathode 36' of tube 35 and resistor lil' is connected between grid 3l and cathode 35. Similarly, a battery 40' is used to supply screen grid potential for tube 35. The aforementioned bucking current is introduced into meter 25 by resistor t2 connected in series with battery @3f across the terminals of the meter.

The potentials appliedtodynodes23 and cathode I'6" 'of device 20 are obtained, as before, from voltage divider 30.

In the circuit of Fig. 3, a meter 25 of greater 'sensitivity than meter 25 of the circuit of Fig. 2 :is required as the current through the meter is only the output current of device 20', whereas, in Fig. 2, the meter current is the output current Aof phototube 20 `amplified by control tube 35. Thereby, in the arrangement shown in Fig. 3 a less sensitive meter may be used than the meter '25 in Fig. 3. Otherwise, both circuits operate in the same manner.

A typical set of values obtained by using the circuit of Fig. 2 with ten 13,000 ohms individual .series connected resistors 30 comprising the voltage divider, and al a 50,000 ohm resistor is indicated in the following Table III:

Table III Meter Out- Sample put Current Density (Relative (Values) A voltage stabilized 15 C. P. incandescent lamp at a distance of four inches from cathode I6 of phototube was used as the light source, and condenser lens I8 was omitted. On the particular meter used, 100 divisions on the uniformly graduated scale thereof corresponded to an output current of ve milliamperes and the Zero mark on the scale corresponded to an output current of three milliamperes. These values are indicated by the curve l5 in the graph shown in Fig. 4. The foregoing figures are representative only and are given merely to illustrate a practical application of the principles of the invention.

It will be noted that the invention provides Ja, simple, effective and stable circuit for obtaining vdirect readings of density on a meter having a -uniformly graduated scale, through the medium .of a photomultiplier tube. The source of current need not necessarily be direct current, but may 'be alternating current. As previously stated, the

principles of the invention are applicable not only to radiant energy excited multiplier tubes, but also tto other types of electron multiplier tu'bes. Thus, grid controlled voltage excited electron multiplier :tubes can be incorporated in circuits designed in accordance with the principles of the present inyention to obtain direct readings, on a uniformly graduated meter scale, of electrical values which are longarithmically related.

While specific embodiments of the invention have been described in detail to illustrate the application of the principles thereof, it will be apparent to those skilled in the art that the invention may be otherwise embodied without departing from such principles.

What is claimed is:

1. A logarithmically responsive electronic circuit, comprising in combination, an electron multiplier tube including a plurality of dynode stages; a source of pontential; means for supplying gradzuated potentials to successive dynode stages; and

, Y 8 means for controlling the effective potentials between successive dynode stages in inverse relationship to the anode current of said multiplier tube, said means comprising variable impedance means in series between said source of potential and said means for supplying graduated potentials, and a circuit responsive to said anode current connected to said impedance means.

2. A logarithmically responsive electronic circuit, comprising in combination, an electrostatically focused electron multiplier tube including a plurality of dynode stages; a source of potential; means for supplying graduated potentials to successive dynode stages; and means for controlling the effective potentials between successive dynode stages in inverse relationship to the anode current of said multiplier tube, said means comprising variable impedance means in between said source of potential and said means for supplying graduated potentials, and a circuit responsive to said anode current connected with said impedance means for varying the effective impedance thereof.

3. A logarithmically responsive electronic circuit, comprising in combination, a photo responsive electron multiplier tube including a plurality of dynode stages; a source of potential; means for supplying graduated potentials to successive dynode stages; and means for controlling the effective potentials between successive dynode stages in inverse relationship to the anode current of said multiplier tube, said means comprising variable impedance means connected in series between said source of potential and said means for supplying graduated potentials, and means responsive to said anode current connected to the output of said multiplier tube and with said variable impedance means whereby the effective impedance of said last-mentioned means is varied in accordance with the current in said anode current responsive means.

4. A logarithmically responsive electronic cir-- cuit, comprising in combination, a photo responsive electrostatically focused electron multiplier tube including a plurality of dynode stages; a source of potential; means for supplying graduated potentials to successive dynode stages; and. means for controlling the effective potentials between successive dynode stages in inverse rela-- tionship to the anode current of said multiplier tube, said means comprising variable impedance element connected in series between said source of potential and said means for supplying graduated potentials, an output circuit for said multiplier tube including output current responsive means and said variable impedance means.

5. A logarithmically responsive electronic circuit, comprising in combination, an electrostatically focused electron multiplier tube including a plurality of dynode stages; a source of potential; means for supplying graduated potentials to successive dynode stages; and means for controlling the effective potentials between successive dynode stages in inverse relationship to the anode current of said multiplier tube, said means comprising a vacuum tube having anode, cathode and control electrodes, an input circuit between said control electrode and said cathode including an impedance element responsive to the anode current of said multiplier tube, an output circuit between anode and .cathode of said vacuum tube including said source of potential and said means for supplying graduated potentials substantially in series. A

6. A logarithmically responsive electronic circuit, comprising in combination, an electron multiplier tube including a plurality oi dynode Stages; a source of potential; means for supplying graduated potentials to successive dynode stages; and means for controlling the effective potentials between successive dynode stages in inverse relationship to the anode current oi said multiplier tube, said means comprising a control vacuum tube having anode, cathode and control electrodes, a circuit between said anode and said cathode including said means for supplying graduated potentials in the form of a resistance element having a plurality of portions connected to respective dynode stages, and said source of potentials substantially in series; a circuit between i said control electrode and said cathode including a grid resistance common to the output circuit of said multiplier tube whereby the grid voltage of said control tube is determined by the anode current in said grid resistance and the conductivity of said tube controlled by said grid voltage determines the effective dynode voltages developed across portions of said resistance element.

7. A logarithrnically responsive electronic circuit comprising in combination, an electrostatically focused electron multiplier tube including anode and intermediate multiplier electrodes; a source of potential; a voltage divider including a plurality of series resistors connected to said intermediate multiplier electrodes, and means for ;,y

modulating the potentials between successive multiplier electrodes in inverse relationship to the anode current of said mutiplier tube, said means comprising a control tube having anode, cathode and control electrodes, a circuit between said anode and said cathode including said voltage divider and said source of potential substantially in series; a circuit between said control electrode and said cathode including a grid resistance common to the output circuit oi said multiplier tube whereby the grid voltage of said control tube is determined by the anode current in said grid resistance and the conductivity of said tube controlled by said grid voltage determines the effective dynode voltages developed across said series resistors and a current indicating meter in the output circuit of said control tube the indication or" which is directly proportional to the current in said Voltage divider.

8. A logarithmically responsive electronic circuit comprsing in combination, an electrostatically focused electron multiplier tube including anode and intermediate multiplier electrodes; a source of unidirectional potential; a voltage divider including a plurality of series resistors each connected to one of said intermediate multiplier electrodes, and means for controlling the eiective potentials between successive multiplier electrodes in inverse relationship to the anode current of said multiplier tube, said means comprising a control having anode, cathode and control electrodes, a circuit between said anode and said cathode including said voltage divider and said source of potential substantially in series; a circuit between said control electrode and said cathode including a grid resistance common to the output circuit of said mutiplier tube whereby the grid voltage of said control tube is determined by the anode current in said grid resistance and the conductivity of said tube, controlled by said grid voltage, determines the effective dynode voltages developed across said series resistors and a current indicating meter in the output circuit of said multiplier tube the indication of which is directly proportional to the current in said grid resistance.

MONROE HAMILTON SWEET.

REFERENCES CITED The following references are of record in the ille of this patent:

UNTED STATES PATENTS Number Name Date 2,093,166 Flory et al. Sept. 14, 1937 2,227,031 Schlesinger Dec. 31, 1940 2,227,095 Liebrnann Dec. 31, 1940 2,231,697 Zworykin et al Feb. 11, 1941 2,290,775 Snyder, Jr July 2l, 1942 2,361,549 Kott Oct. 31, 1944 2,407,564 Martin et al Sept. 10, 1946 2,417,023 Sweet Mar. 4, 1947 OTHER REFERENCES Larson et al., pages 226, 229, July 1940, RSI.

Article by John Russel, R. S. I., December 1937, pages 495-6, vol. 8.

A Precision Direct-Reading Densitometer, an article by M. 1-1. Sweet in Journal of the Society of Motion Picture Engineers for Feb. 1942, pages 148-172 cited; pages 153-160 especially relied upon. 

